This invention relates generally to a Hybrid Scavengeless Development (HSD) apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a method to prevent toner or other particulate contamination of wires in such an HSD developer unit.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the receiver, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or noninteractive development systems.
Hybrid scavengeless development technology develops toner via a conventional magnetic brush onto the surface of a donor roll and a plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50-200 .mu.m) partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor core to produce the field necessary for toner development. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical ac voltages of the wires relative to the donor are 700-900 Vpp at frequencies of 5-15 kHz. These ac signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
A problem with developer systems using wires is that a "Wire History" develops which involves highly charged (though sometimes low charged) and generally small toner or other particles being attracted to the wire and sticking to the wire as a result of either adhesive or image attractive forces. One of the primary drivers of wire history has been found to be the DC offset of the wires relative to the donor roll and the toner layer. It has been observed that if the wires are DC biased electrically attractive to the toner relative to the "toner layer potential" level that wire history is made significantly worse.